The present invention generally relates to a mixing device for a viscous liquid and more particularly, relates to a static mixer for a viscous liquid that is constructed of an elongated cylindrical tank, an elongated cylindrical mixing sleeve inside the tank, a plurality of curvilinear tubes for feeding the viscous liquid, a spiral plate for guiding the liquid flow, and an outlet tube for outputting a mixture of the viscous liquid and a solvent.
Apparatus for polishing thin, flat semi-conductor wafers is well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semi-conductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semi-conductor wafer during the fabrication of semi-conductor devices on the wafer. A wafer is xe2x80x9cplanarizedxe2x80x9d or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The apparatus 10 for chemical mechanical polishing consists of a rotating wafer holder 14 that holds the wafer 10, the appropriate slurry 24, and a polishing pad 12 which is normally mounted to a rotating table 26 by adhesive means. The polishing pad 12 is applied to the wafer surface 22 at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing a metal oxide may be formed and removed repeatedly.
A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel. It is known in the art that uniformity in wafer polishing is a function of pressure, velocity and the concentration of chemicals. Edge exclusion is caused, in part, by non-uniform pressure on a wafer. The problem is reduced somewhat through the use of a retaining ring which engages the polishing pad.
In the polishing operation shown in the enlarged cross-sectional view of FIG. 1B, the slurry solution 24 must be forced into an interface between the wafer 10 and the polishing pad 12 in order for the chemical reaction and the mechanical removal process 20 to operate efficiently. The slurry solution 24 (also shown in FIG. 1A) is dispensed from a dispensing nozzle (not shown) onto the polishing pad 12. In most commercial CMP apparatus, the slurry solution 24 is stored in a reservoir and delivered to the dispensing nozzle through a conduit. The slurry solution stored in the reservoir and in the delivering conduit is not provided with a temperature control device. The slurry solution 24 is normally applied to the polishing pad 12 at the same temperature as the chamber temperature in the CMP apparatus, i.e., approximately at room temperature.
The slurry solution is normally delivered by a commercial supplier in a preset concentration which must be diluted by a solvent such as ultra-pure water (UPW) for a specific CMP process. Conventionally, the dilution of the viscous slurry solution by a solvent can be accomplished in a dynamic mixer utilizing a propeller type mixing blade for producing a turbulent flow in the solution to achieve mixing. The mixing efficiency achieved by the dynamic mixer is poor and furthermore, the dynamic mixer frequently occupies a large floor space.
More recently, static mixers have been used which utilize a static mixing mechanism without using any mechanical moving parts. In a typical static mixer, the mixer and the mixing tank assembly are combined into one unit to minimize the floor space used in a factory. Factory space utilization has become more important in the present deep-sub-micron fabrication environment, since global planarization has become an essential process for achieving high IC device density on the chips fabricated. In a global planarization process that is carried out by chemical mechanical polishing, slurries are adapted as an abrasive material for producing the planarization effect. For cost reduction purposes, it has become more effective to dilute concentrated slurry solutions with solvents, i.e. ultra-pure water. The slurry blending requirements vary from process to process. However, a slurry mixer must blend slurry in large volumes to a tool-specific blend ratio. In other words, a high mixing efficiency and a reliable viscous liquid mixer are critical requirements in the present deep-sub-micron fabrication environment. For instance, a slurry mixer may be used to blend either a two-component slurry, i.e. a tungsten slurry that has different pH values. When a mixing process is not properly conducted, a slurry of unstable quality is obtained which may lead to increased crystallization of the slurry solution and an increasing probability of pH shock.
Referring now to FIG. 1C, wherein a typical static mixer utilized in slurry dilution for a CMP process is shown. The static mixer system 30 is constructed of a mixing tank 32 containing a cavity 34 therein for conducting the mixing process. The mixing tank 32 is essentially empty in the cavity 34 with no mixing aid built therein. Into the cavity 34, is fed through an to inlet conduit 36 a slurry solution from a slurry conduit 38 fed by a slurry pump 40, an oxidizer solution 42 (used in a tungsten CMP process) fed by an oxidizer pump 44, and ultra-pure water through a water conduit 46 fed by a water pump 48. Simultaneously, a recirculated slurry solution is fed into the inlet conduit 36 by a recirculating pump 50 through conduit 52 for recycling the slurry solution 54 contained in the cavity 34. On the side of the static mixer tank 32, is provided a sight tube 56 including a plurality of level sensors 58.
While the static mixer 30 shown in FIG. 1C provides some benefits over the dynamic mixer previously utilized, the mixing tank 32 does not provide high efficiency mixing for the slurry solution 54 contained in the cavity 34.
It is therefore an object of the present invention to provide a static mixer for a viscous liquid that does not have the drawbacks or shortcomings of the conventional static mixers.
It is another object of the present invention to provide a static mixer for a viscous liquid that can be designed with a minimal utilization of floor space in a semiconductor fabrication facility.
It is a further object of the present invention to provide a static mixer that has improved construction inside the mixing tank for achieving high efficiency mixing.
It is another further object of the present invention to provide a static mixer for a viscous liquid that has a specially designed mixing chamber inside the mixing tank.
It is still another object of the present invention to provide a static mixer for a viscous liquid that utilizes a mixing tank equipped with an elongated cylindrical mixing sleeve positioned inside the tank.
It is yet another object of the present invention to provide a static mixer for a viscous liquid wherein the mixing tank is equipped with an elongated cylindrical mixing sleeve and a plurality of curvilinear liquid feeding tubes situated on top of the tank cavity.
It is still another further object of the present invention to provide a static mixer for a viscous liquid that is equipped with a mixing tank constructed with a spiral plate along an inside surface of the tank sidewall extending continuously from a top of the cavity to a bottom of the cavity.
It is yet another further object of the present invention to provide a static mixer for a viscous liquid wherein a mixing tank is constructed with an elongated cylindrical mixing sleeve, a plurality of curvilinear feed tubes, a spiral plate positioned on an inside surface of the sidewall of the tank, and an outlet tube situated at a bottom of the tank and inside the mixing sleeve for outputting a diluted mixture of the viscous liquid.
In accordance with the present invention, a static mixer for diluting a viscous liquid by a solvent for use in a semiconductor fabrication process is disclosed.
In a preferred embodiment, a static mixer for a viscous liquid is provided which includes an elongated cylindrical tank that has a cavity and a first diameter defined by a tank wall; an elongated cylindrical mixing sleeve that has a second diameter smaller than the first diameter situated inside the tank cavity, the mixing sleeve has a multiplicity of mixing apertures therethrough; a plurality of curvilinear tubes situated near a top wall of the tank for flowing a mixture of the viscous liquid and a solvent into the tank cavity, each of the plurality of curvilinear tubes has an outlet extending outwardly away from a center of the tank cavity toward an interior surface of the tank wall generating a spiral flow of the mixture; a spiral plate that has a predetermined width positioned on the interior surface of the tank wall extending continuously from the top wall to a bottom wall of the tank; and an outlet tube situated at the bottom of the tank and inside the mixing sleeve for outputting the mixture.
The static mixture for a viscous liquid may further include a plurality of level sensors for sensing a level of the mixture or a sight tube mounted on a sidewall of the elongated cylindrical tank for observing a level of the mixture. The static mixer may further include a mixing sleeve mounted in an upright position inside the tank cavity when the elongated cylindrical tank is mounted in an upright position. The multiplicity of mixing apertures in the mixing sleeve may have a diameter between about 3 mm and about 15 mm, or more preferably a diameter between about 5 mm and about 10 mm. The second diameter of the mixing sleeve may be at least 2 cm smaller than the first diameter of the elongated cylindrical tank. The plurality of curvilinear tubes may be arranged such that each two of the tubes is in an S-shape with two outlets pointing toward the interior surface of the tank wall.
In the static mixer for a viscous liquid, the spiral plate may be arranged along the interior surface of the tank wall for guiding the spiral flow of the mixture to the bottom wall of the tank and to enter the mixing sleeve through the mixing apertures. The predetermined width of the spiral plate may be less than 10% of the first diameter of the elongated cylindrical tank, or may be less than 2 cm. The static mixer may further include a pump means in fluid communication with the outlet tube for withdrawing the mixture from the tank cavity. The plurality of curvilinear tubes may further include an inlet in fluid communication with at least two inlet conduits, wherein the at least two inlet conduits may be a slurry feed conduit and a solvent feed conduit. The at least two inlet conduits may be three conduits of a slurry feed conduit, a solvent feed conduit and a recirculating slurry conduit. The plurality of curvilinear tubes may be fixedly mounted to the top wall of the tank. The spiral plate may be fixedly mounted on the interior surface of the tank wall by welding means. The plurality of curvilinear tubes may be at least four tubes each having an outlet pointing toward the interior surface of the tank wall.